1. Field of the Invention
The present invention relates to two stage subterranean zone fracturing fluids and methods of fracturing subterranean zones with the fracturing fluids.
2. Description of the Prior Art
Hydrocarbon producing wells are often stimulated by hydraulic fracturing treatments. In such treatments, a viscous fracturing fluid which also functions as a carrier fluid is pumped into a producing zone to be fractured at a rate and pressure such that one or more fractures are formed in the zone. Proppant particles for propping the fractures open are suspended in at least a portion of the fracturing fluid so that the proppant particles are deposited in the fractures when the fracturing fluid is broken, i.e., the viscosity of the fluid is reduced. The breaking of the fracturing fluid is accomplished by a viscosity breaker in the fracturing fluid whereby the fracturing fluid reverts to a thin fluid which is returned to the surface. The proppant particles deposited in the fractures when the fracturing fluid viscosity is reduced function to prevent the fractures from closing so that conductive channels are formed through which produced hydrocarbons can readily flow.
While the fracturing fluids utilized heretofore have generally achieved good results, problems involving the reduction in the permeabilities of the fractured formations often occur. For example, the fracturing fluids have leaked off into the fractured formation walls whereby the permeabilities of the fractured formations are reduced and have allowed proppant particles to settle in the fractures prior to when the viscosities of the fracturing fluids are reduced. When the fracturing fluids are broken residue is often produced which enters and reduces the permeabilities of the proppant packs formed in the fractures. Finally, the recovery of the fracturing fluids after their viscosities are broken has often been difficult.
Foamed fracturing fluids have heretofore been utilized for their efficient creation of fractures. Because of the two phase character of the foamed fluids, when they enter high permeability zones, they block further flow into that zone. Foamed fracturing fluids are also known for their fluid loss control whereby a minimum thickness of gelled filter cake must be cleaned up later. Once the pumping of foamed fracturing fluids has ceased and the pressure exerted on the fluids is reduced, the foam expands and drives the liquid portions of the fracturing fluids into the well bore and to the surface for recovery. While the foams have excellent properties for forming and cleaning up fractures as described above, they are limited in their ability to contain high concentrations of proppant particles.
Hydraulic fracturing fluids prepared from liquids, polymers and cross-linking agents are commonly utilized as a result of their excellent proppant particle carrying ability. Also, as a result of the high viscosity of the fluids, they create wide fractures that can accept high concentrations of proppant particles. However, when such cross-linked fracturing fluids are used, relatively thick filter cake is deposited on the fracture faces which must be broken and removed in order to prevent the filter cake from entering and reducing the permeability of the proppant packs in the fractures when the well is produced. The filter cake on the fracture faces contains a high concentration of polymers and it is very difficult to dissolve and remove.
Thus, there are needs for fracturing fluids that have excellent fracture creating and clean up properties and are able to carry high quantities of proppant particles into the fractures.